Plain Text Font Size

I have set Zimbra Desktop to compose in plain text, as I do with other mail clients. However, I can see no way to change either the font or the size while in plain text compose mode (or read mode). Is there a way to do that? I think right now that is a shortcoming. In many other ways, I really like Zimbra Desktop. On a trial basis, I am using it both both my secondary mail (Yahoo) and Primary mail (Gmail) IMAP accounts.